A stent, as illustrated in FIG. 1, is an intravascular prosthesis that is delivered and implanted within a patient's vasculature or other bodily cavities and lumens by a balloon catheter. The structure of a stent is typically composed of scaffolding, substrate, or base material that includes a pattern or network of interconnecting structural elements often referred to in the art as struts or bar arms. Referring to FIG. 1, an exemplary stent 14 is illustrated. Stent 14 can include a plurality of struts 16 connected by linking struts 11, with interstitial spaces 18 located in between the struts. The plurality of struts 16 can be configured in an annular fashion in discrete “rows” such that they form a series of “rings” throughout the body of stent 14. Stents can be used in percutaneous transluminal coronary angioplasty (PTCA) or percutaneous transluminal angioplasty (PTA). Conventional stents and catheters are disclosed by U.S. Pat. Nos. 4,733,665, 4,800,882, 4,886,062, 5,514,154, 5,569,295, and 5,507,768. In advancing a stent through a body vessel to a deployment site, the stent must be able to securely maintain its axial as well as rotational position on the delivery catheter without translocating proximally or distally, and especially without becoming separated from the catheter. Stents that are not properly secured or retained to the catheter may slip and either be lost or be deployed in the wrong location. The stent must be “crimped” in such a way as to minimize or prevent distortion of the stent and to thereby prevent abrasion and/or reduce trauma to the vessel walls.
Generally, stent crimping is the act of affixing the stent to the delivery catheter or delivery balloon so that it remains affixed to the catheter or balloon until the physician desires to deliver the stent at the treatment site. Current stent crimping technology is sophisticated. Examples of such technology which are known by one of ordinary skill in the art include a roll crimper; a collet crimper; and an iris or sliding-wedge crimper. To use a roll crimper, first the stent is slid loosely onto the balloon portion of the catheter. This assembly is placed between the plates of the roll crimper. With an automated roll crimper, the plates come together and apply a specified amount of force. They then move back and forth a set distance in a direction that is perpendicular to the catheter. The catheter rolls back and forth under this motion, and the diameter of the stent is reduced. The process can be broken down into more than one step, each with its own level of force, translational distance, and number of cycles. This process imparts a great deal of shear to the stent in a direction perpendicular to the catheter or catheter wall. Furthermore, as the stent is crimped, there is additional relative motion between the stent surface and the crimping plates.
The collet crimper is equally conceptually simple. A standard drill-chuck collet is equipped with several pie-piece-shaped jaws. These jaws move in a radial direction as an outer ring is turned. To use this crimper, a stent is loosely placed onto the balloon portion of a catheter and inserted in the center space between the jaws. Turning the outer ring causes the jaws to move inward. An issue with this device is determining or designing the crimping endpoint. One scheme is to engineer the jaws so that when they completely close, they touch and a center hole of a known diameter remains. Using this approach, turning the collet until the jaws abut each other crimps the stent to the known outer diameter. While this seems ideal, it can lead to problems. Stent struts have a tolerance on their thickness. Additionally, the process of folding non-compliant balloons is not exactly reproducible. Consequently, the collet crimper exerts a different amount of force on each stent in order to achieve the same final dimension. Unless this force, and the final crimped diameter, is carefully chosen, the variability of the stent and balloon dimensions can yield stent or balloon damage.
In the sliding wedge or iris crimper, adjacent pie-piece-shaped sections move inward and twist, much like the leaves in a camera aperture. This crimper can be engineered to have two different types of endpoints. It can stop at a final diameter, or it can apply a fixed force and allow the final diameter to float. From the discussion on the collet crimper, there are advantages in applying a fixed level of force as variability in strut and balloon dimension will not change the crimping force. The sliding wedges impart primarily normal forces. As the wedges slide over each other, they impart some tangential force. Lastly, the sliding wedge crimper presents a nearly cylindrical inner surface to the stent, even as it crimps. This means the crimping loads are distributed over the entire outer surface of the stent.
All current stent crimping methods were developed for all-metal stents. Stent metals, such as stainless steel, are durable and can take abuse. When crimping is too severe, it usually damages the underlying balloon, not the metal stent. But polymeric stents present different challenges. A polymer stent requires relatively wider struts than metal stents so as to provide suitable mechanical properties, such as radial strength. At the crimping stage, less space is provided between the struts which can result in worse stent retention than a metallic stent. Moreover, the use of high processing temperature during the crimping process to enhance stent retention may not be possible as a polymeric stent may have a glass transition temperature between 40-60 degrees Celsius. Higher processing temperatures may cause the stent to lose some of its preferred mechanical properties.
Polymeric stents can also be more susceptible to crack propagation during crimping or expansion by a balloon. When a polymeric stent is placed on a balloon and a crimping pressure applied, the load on the struts can vary significantly. A significant cause for this load variation is the balloon surface, which due to its variation in surface geometry and stiffness properties over the surface imposes non-uniform reaction forces to crimping against the stent luminal surface. As the stent is pressed into the balloon surface, different stent struts will experience different loadings because the balloon does not everywhere have the same stiffness properties, i.e., some areas are less compliant than others. Moreover, the balloon surface is generally not smooth. As such, bumps or mounds over the balloon surface, especially in areas of relatively high stiffness, can produce different loadings across the stent body. For example, when a stent is crimped on a non-compliant balloon having folded wings or pleats, there can be noticeable bumps or mounds in the areas where the material is folded. Moreover, the stiffness in these areas, i.e., where the balloon material is folded over itself, can be noticeable higher than in areas distal from the folds. Variations in the surface geometry of the balloon and/or stiffness about the circumference and/or length of the balloon can lead to high stress concentrations resulting in twisting, bending, warping of individual struts or segments of the stent. Excessive loading in areas near corners, especially corners having surface imperfections created during the stent making process (typically a high stress area), can result in propagation of micro cracks leading to a significant reduction in strength or outright failure of a strut when the stent is expanded to its full diameter by the balloon.
The present invention provides a novel method of crimping a stent, more specifically a polymeric stent on an expandable member or a balloon, and a novel apparatus for delivery of a stent on a balloon catheter.